Regenerated cellulose fiber

ABSTRACT

The present invention relates to a regenerated cellulose fiber, which is characterized by the combination of the following features:
         the fiber has in its dry condition a collapsed hollow cross-sectional structure   the fiber has in its wet condition a cross-sectional structure with cavities   the fiber is segmented in the longitudinal direction by dividing walls   there is incorporated into the fiber an absorbent polymer, in particular carboxymethylcellulose.
 
The fiber may be obtained by a process, wherein there is admixed a carbonate as well as an absorbent polymer, in particular carboxymethylcellulose, to a viscose dope.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a regenerated cellulose fiber preparedby the viscose process.

In particular the invention relates to a regenerated cellulose fiberwith a collapsed hollow cross-sectional structure.

For sanitary uses such as, for example, tampons or absorbent cores ingeneral, fibers are desirable, which have an especially high waterstoring capacity in order to in this way allow for a possibly highabsorption capacity of the sanitary product.

Conventional super-absorbent materials have the disadvantage that theyusually form a gel upon contact with water and thus reduce the integrityof the absorbent core and, furthermore, block the further transport ofliquid through the absorbent core. The integrity of the absorbent core,hence, is important in these applications, as there is prevented, bymeans of this material, the exit of the absorption material and thus thecontamination of, for example, wounds.

It is also desired for wound dressings that the absorbing material doesnot agglutinate with the wound so that it is achievable to remove thedressing from the wound without causing pain.

Another demand for the fiber material in these fields of application isthe flawless processability in conventional production techniques forthe fleece production.

Viscose fibers with a high portion of carboxymethylcellulose (CMC) havebeen known. This is a mixed fiber that is obtained by spinning incarboxymethylcellulose into the viscose dope. Such fibers have also beencommercially produced (U.S. Pat. No. 4,199,367 A, U.S. Pat. No.4,289,824 A).

The production of viscose fibers with a hollow cross-sectional structure(“hollow fibers”) by spinning in sodium carbonate into the viscose dopehas already been known since 1920. Fibers of this type were previouslycommercially produced by various producers. Hollow fibers of the“superinflated” type with high water retention capacity have beenproduced especially for sanitary applications (GB 1,333,047 A, GB1,393,778 A).

A comprehensive survey on development and history of hollow viscosefibers may be found with: C. R. Woodings, A. J. Bartholomew; “Themanufacture properties and uses of inflated viscose rayon fibers”; TAPPINonwovens Symposium; 1985; pp. 155-165.

Viscose-CMC mixed fibers show in their production the disadvantage thatdue to their high absorbance and the strong adherence of the fibers toeach other in a wet condition (gel effect on the fiber surface) thefibers in the conventional viscose process, from an integration of about15% CMC based on the cellulose on, tend to sink in the baths of thesubsequent treatment and not to float like conventional viscose fibers.This substantially hinders the production of these fibers in a commoncommercial process.

A desired effect of these fibers is their gel-like surface consistencyin a wet condition. This effect in touch, however, is partly reduced bythe fact that these fibers have an extremely grooved surfacestructure—like conventional viscose fibers.

The maximum increase of the water retention capacity of such fibers isrestricted, as during the spinning in of >50% CMC there arise greatproblems in the processing step thereof.

Also with hollow viscose fibers the increase of the water retentioncapacity is limited. Due to a stronger inflation of the fiber, therewill not be obtained any higher water retention capacity anymore from acertain level on, and there are rather formed band-like fibers with aflat cross-section having low water retention capacity, wherein thepost-treatment thereof in a staple fiber method is rather associatedwith problems due to their large and smooth surface and the highfiber-fiber binding capacity associated therewith.

There are further known carboxymethylated fibers, i.e. fibers that havebeen produced by subsequent carboxymethylation of viscose fibers(commercially available as “Aquacell” fibers of the company Convatec;U.S. Pat. No. 6,548,730 B1; AU 757461 B; WO00/01425 A1; EP 1 091 770A1). The fibers allow for a high amount of water absorption but formupon contact with water a gel with simultaneous complete loss of thefiber structure, which is not desirable for all possible applications.

The U.S. Pat. No. 3,318,990 describes a method for the production offlat viscose fibers, wherein there is admixed to the viscose, on the oneside, sodium carbonate and, on the other side, a high-molecularsubstance swelling in water, e.g., CMC.

The resulting fibers are described as completely flat. The collapsedcross-sectional structure retains its form also in a wet condition. Thefibers are suitable for the production of paper according to U.S. Pat.No. 3,318,990.

SUMMARY OF THE INVENTION

The present invention has the objective to provide a viscose fiber witha possibly high water retention capacity, a possibly high absorptionunder pressure and an additional surface gel effect. The fiber is to beproduced in the conventional methods for the production of viscosefibers and to be processed according to the conventional method for theproduction of fleece (for example, carding, solidification by needling,hydro-entanglement, thermal solidification, calandration).

This task is solved by a regenerated cellulose fiber which ischaracterized by the combination of the following features:

-   -   the fiber has in its dry condition a collapsed hollow        cross-sectional structure    -   the fiber has in its wet condition a cross-sectional structure        with cavities    -   the fiber is segmented in the longitudinal direction by dividing        walls    -   there is incorporated into the fiber an absorbent polymer.

The cellulose fibers are produced by

-   -   providing a viscose dope    -   admixing a carbonate compatible with viscose into the viscose        dope    -   admixing an absorbent polymer into the viscose dope    -   spinning the viscose dope in a spinning bath, thereby forming        fibers, wherein    -   the degree of ripening of the viscose dope before spinning is        less than 15° Hottenroth, preferably 10° to 14° Hottenroth,    -   the content of H₂SO₄ in the spinning bath is 8% to 10%    -   the content of ZnSO₄ in the spinning bath is 0.3% to 0.5% and    -   the content of Na₂SO₄ in the spinning bath is 25% to 30%.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the longitudinal section of a dried cellulose fiberaccording to the invention.

FIG. 2 shows a much enlarged longitudinal section of a dried cellulosefiber according to the invention.

FIG. 3 shows the cross-section of dried cellulose fibers according tothe invention.

FIG. 4 shows the tubular cross-section of a cellulose fiber according tothe invention upon contact with water.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a regenerated cellulose fiber with acollapsed hollow cross-sectional structure, which is characterized bythe combination of the following features:

-   -   the fiber has in its dry condition a collapsed hollow        cross-sectional structure    -   the fiber has in its wet condition a cross-sectional structure        with cavities    -   the fiber is segmented in the longitudinal direction by dividing        walls    -   there is incorporated into the fiber an absorbent polymer.

The absorbent polymer is preferably carboxymethylcellulose (CMC). Othersuitable absorbent polymers may be selected from the group of polymersthat are compatible with the viscose process, such as, for example,viscose carbamate, carboxymethylcellulose, alginates or homo- andco-polymers of acrylic acids (such as, for example, described in U.S.Pat. No. 4,399,255).

By means of the present invention, there is, for the very first time,provided in the preferred embodiment a combination of a hollow viscosefiber, which has a tubular structure with cavities again uponre-wetting, and a viscose mixed fiber with CMC.

The expert understands that a regenerated cellulose fiber with collapsedhollow cross-sectional structure is a viscose fiber, the cross-sectionof which has been so strongly inflated that the cross-sectionalstructure collapses in itself. The production of such fibers (throughadmixture of sodium carbonate) is described in GB 1,333,047 A and GB1,393,778 A.

The fiber according to the invention has in its dried conditionpreferably an irregular multilobal cross-section or an irregularlygrooved cross-section. Such fibers are also designated as “SI” fibers(for superinflated). In contrast to the fibers described in the U.S.Pat. No. 3,318,990 A the cross-section will re-open upon wetting of thefibers and it will regain its tubular form, with cavities being formed.

The fiber according to the invention is different to the fibersdescribed in U.S. Pat. No. 3,318,990 A in particular due to the presenceof a segmentation in the longitudinal direction as a result of dividingwalls.

The distance between the dividing walls thus is preferably between 0.3and 3 times, preferably 0.5 to 2 times, the fiber width averaged overthe fiber length. There is assumed that the presence of these dividingwalls is in part responsible for the fact that upon wetting of thefibers, there is re-formed a tubular structure with cavities.

This tubular structure is probably responsible for the fibers accordingto the invention having excellent absorbance compared to the fibers ofthe U.S. Pat. No. 3,318,990 that remain flat also upon re-wetting.

By the measure that there is incorporated an absorbent polymer, inparticular CMC; into the fiber the expert understands as beingintegrated, in the matrix of the (upon regeneration of the fiber)underivatized cellulose, the absorbent polymer, for example CMC. Thisis—in contrast to the application of CMC onto the already fabricatedfiber—possible, especially, by spinning in CMC into the viscose dope.

The cellulose fiber according to the invention has a very high waterretention capacity; the fiber character, however, still remains the samein its wet condition. Furthermore, there has been shown that the fiberaccording to the invention, in contrast to conventional viscose-CMSmixed fibers, may be well produced and in particular processed in aconventional viscose process.

The portion of absorbent polymer, in particular of CMC, in the fiberaccording to the invention is preferably 5 to 50% by weight, especiallypreferably 15 to 40% by weight, most preferably 20 to 30% by weight,based upon the underivatized cellulose.

The cellulose fiber according to the invention is preferably availablein the form of a staple fibers and may be produced in all common titerareas. Preferably, the fiber titer may be 0.5 dtex to 8 dtex, preferablyfrom 1.3 to 6 dtex.

The fiber length of the fiber according to the invention may be from 2mm to 80 mm and is dependent in particular on the field of application.For a wet-laid process, there are suitable in particular fiber lengthsof 2 to 20 mm, for a carding process, fiber lengths from 20 to 80 mm aresuitable.

The tenacity of the fiber according to the invention is typically above10 cN/tex; the tear elongation is above 15%.

The cellulose fiber according to the invention has preferably a waterretention capacity of at least 300%, measured according to DIN 53814.

There was, for example, measured in the fibers according to theinvention with a portion of 35% by weight CMC a water retention capacityof up to 400% WRC.

A cylindrical tampon prepared from the fiber according to the inventionwith a mass of 2.72 g, a length of 44 mm and a diameter of 13 mm haspreferably a Syngina value of at least 5.2 g/g, measured according toEDANA/INDA Standard Test Methods for the Nonwovens and RelatedIndustries ERT 350.0 or WSP 350.1, respectively.

In this way, the fiber has a higher Syngina value than the currentlycommercially most important absorbent viscose fiber, a viscose fiberwith a regular trilobal cross-section marketed under the trade name“Galaxy”® (EP 0 301 874 A1).

For the production of a regenerated cellulose fiber according to thepresent invention, there is used a process, comprising the steps

-   -   providing a viscose dope    -   admixing a carbonate compatible with viscose into the viscose        dope    -   admixing of an absorbent polymer into the viscose dope    -   spinning the viscose dope in a spinning bath, thereby forming        fibers, wherein    -   the degree of ripening of the viscose dope before spinning is        less than 15° Hottenroth, preferably 10° to 14° Hottenroth    -   the content of H₂SO₄ in the spinning bath is 8% to 10%    -   the content of ZnSO₄ in the spinning bath is 0.3% to 0.5% and    -   the content of Na₂SO₄ in the spinning bath is 25% to 30%.

The process according to the invention, hence, combines the so far knownstep of spinning in a carbonate, in particular sodium carbonate, forproducing a hollow cross-section, with the step of spinning in anabsorbent polymer.

In comparison with the method described in U.S. Pat. No. 3,318,990,there is spun in the process according to the invention a viscose with alower degree of ripening (expressed in Hottenroth) and the spinning bathhas a smaller portion of H₂SO₄ and ZnSO₄. There is made the assumptionthat these different procedures result in the differences mentionedabove between the fiber according to the invention and the fiberdescribed in the U.S. Pat. No. 3,318,990, in particular in terms ofcross-sectional structure and also in terms of the othercharacteristics.

The absorbent polymer is preferably CMC. The following embodimentsrelate to this preferred embodiment; they apply, however, mutatismutandis also to other absorbent polymers that are compatible with theviscose process.

As carbonate, there is suitable any carbonate, in particular an alkalimetal carbonate, which is compatible with the conditions of the viscoseprocess. Especially suitable is sodium carbonate. Other suitablecarbonates are potassium carbonate, calcium carbonate as well as ingeneral all carbonates releasing carbon dioxide under the influence ofan acid.

Exerting influence on the cross-sectional form of the fiber (inflationand collapse) in a way so that there is developed a collapsed hollowcross-sectional structure, for example, by selecting the appropriateconditions in the spinning bath, selection of temperature and, ofcourse, amount of carbonate added, is known to the expert in particularfrom the patent publications cited above.

Preferably, the carbonate is admixed in an amount of 11% by weight to23% by weight (calculated as (CO₃)²), based on the cellulose in theviscose dope. In the specific case of sodium carbonate, the preferredamount is from 20% by weight to 40% by weight based on cellulose.

Further preferably, the absorbent polymer, in particular thecarboxymethylcellulose, is admixed in an amount of 5% by weight to 50%by weight, especially preferably 15% by weight to 40% by weight, mostpreferably 20% by weight to 30% by weight, based on the cellulose in theviscose dope.

The carbonate and/or the carboxymethylcellulose may preferably beadmixed in the form of a solution.

In particular, the carboxymethylcellulose may be admixed in the form ofan alkaline solution, containing 2% by weight to 9% by weight,preferably 3% by weight to 5% by weight NaOH and 5% by weight to 15% byweight, preferably 6% by weight to 12% by weight,carboxymethylcellulose.

The carbonate and the carboxymethylcellulose may also be admixedtogether, in particular in the form of a joint solution.

For the production of a cellulose fiber according to the invention theremay also be used a conventional viscose dope.

A typical embodiment of the process according to the invention comprisesthe following measures:

To a viscose dope produced according to conventional methods (cellulosecontent=9-10%, NaOH content 5-6%; viscosity 30-50 falling ball seconds,degree of ripening 10-15° Hottenroth) there are added 25-35% by weightNa₂CO₃ (to be varied according to fiber titer), based on the cellulosein the viscose dope, in the form of a 20% solution of Na₂CO₃. There arefurther added to the dope 5% by weight to 45% by weight, preferably 20%by weight to 30% by weight, based on the cellulose in the viscose dope,carboxymethylcellulose in the form of an alkaline solution (2-9% byweight NaOH; preferably 35% by weight). The concentration of thesolution is 5% by weight to 15% by weight, preferably 8-12% by weight.The carboxymethylcellulose is a commercially available product with adegree of substitution DS of 0.6-1.2, preferably 0.65-0.85 and aviscosity (2% solution; 25° C.) of 30-800 mPas; preferably 50-100 mPas.

The additive solutions (carbonate and CMC) are preferably injected intothe already spinnable viscose dope, in particular immediately before thespinning. The viscose is subsequently homogenized, preferably via adynamic mixer.

The viscose is spun out and subjected to a post-treatment at parameterscommon for textile fibers.

Examples

For the production of regenerated cellulose fibers described in thefollowing there was used a conventional viscose dope.

Experiment A: spinning of the viscose dope without additives

Experiment B: spinning of the viscose dope with 30% Na₂CO₃ based oncellulose

Experiment C (according to the invention): spinning of the viscose dopewith 30% Na₂CO₃ and 20% CMC (DS=0.81; viscosity 2%=53 mPas, Type Blanose7M1F, producer: company Aqualon France (Hercules)), each based on thecellulose present in the viscose dope.

The dopes were spun out into fibers with a titer of 4.4 dtex withidentical spinning parameters.

Experiment D: spinning of the viscose dope without additives

Experiment E: spinning of the viscose dope with 20% CMC (specificationas in experiment C) based on cellulose

Experiment F: spinning of the viscose dope with 30% CMC (specificationas in experiment C) based on cellulose

The dopes were spun out into fibers with a titer of 3 dtex withidentical spinning parameters.

Morphology of the Produced Fibers

Viewed with a microscope, the fibers B and C are hollow fibers of the SItype (superinflated, segmented). The fibers A, D, E and F are fiberswith a grooved and round cross-section that is typical for viscosefibers.

Longitudinal section and cross section of the dried fiber C according tothe invention are illustrated in the FIGS. 1, 2 and 3.

In the FIGS. 1 and 2 there is clearly visible the segmentation in thelongitudinal direction, which is typical for the fiber according to theinvention. The segmentation is the result of dividing walls, which—asmay be recognized in FIG. 2 in an increased magnification—in fact resultin a rigid and membrane-like sub-division.

In FIG. 3 there is visible the collapsed hollow cross-sectionalstructure, which is typical for “SI” fibers, with the formation of aplurality of sides or irregularly grooved cross-sections.

FIG. 4 shows the tubular cross-section of the fiber according to theinvention upon wetting of the dry fiber.

Spinning Performance:

All fibers may be spun with the conventional parameters. Whereas thefibers E and F tend to deposit in the baths of the post-treatment of thefibers, this effect is not observable in the fiber C according to theinvention.

Water Retention Capacity:

For all fibers, the water retention capacity was measured in % accordingto DIN 53814.

Fiber A: 80%

Fiber B: 220%

Fiber C: 335%

Fiber D: 93%

Fiber E: 149%

Fiber F: 193%

As the results show, the water retention capacity in absolute terms maybe increased by spinning in 20% CMC based on the cellulose by about 50%,by spinning in 30% CMC by 100%. (comparison of the fibers D->E->F).

By spinning in 30% Na₂CO₃ based on cellulose, the water retentioncapacity may be increased by 140%. (comparison of the percentage, fiberA->B).

By spinning in 20% CMC and 30% Na₂CO₃ based on cellulose (according tothe invention), the water retention capacity, however, may be increasedby 255%. This shows that the effects of the two additives do no increaseadditively but rather synergistically:

The effect of the addition of CMC alone or Na₂CO₃ alone, respectively,amounts to:

A->B=+140%

D->E=+56%

The expected value for the fiber C, hence, would be 80+140+56=276%

As the measured water retention capacity of the fiber amounting to 335%is higher by the factor 1.2, the two additive components surprisingly,hence, act in a synergistic way.

Gel Effect:

The surface gel effect (slimy and soapy touch) is somehow stronger inthe fibers C according to the invention than in the comparative sample E(with the same content of CMC). This gel effect is only developed in awet condition.

Fiber Values:

The fiber C according to the invention obtains at a titer of 4.4 dtex atenacitiy of 13.1 cN/tex with a tension of 22.9%, which is acceptablefor a use in the field of fleece processing.

Absorption Under Pressure:

From the fibers A, B and C there were pressed tampon plugs, eachamounting to 2.72 g, and there was carried out an Syngina measurementaccording to WSP 351.0. As an additional reference, there was producedan identically produced plug made of 100% commercial trilobal viscosefiber (“Galaxy”®).

Results:

Fiber A: 4.56 g/g

Fiber B: 4.75 g/g

Fiber C: 5.73 g/g

100% Galaxy: 5.05 g/g

The result shows that with the fibers according to the invention theremay be obtained a significantly higher Syngina absorption even incomparison with commercial trilobal fibers.

The regenerated cellulose fiber according to the invention, hence, isespecially suitable for the use in absorbing products, sanitaryproducts, in particular tampons, sanitary aids for incontinence,sanitary napkins and panty liners, packaging of foodstuff, in particularfor meat products, papers, in particular filter papers, clothes (e.g.,clothing textiles for the moisture management in combination with otherfibers or as multi-layered construction) and wound dressings.

1. A regenerated cellulose fiber, wherein: the fiber has in its drycondition a collapsed hollow cross-sectional structure the fiber has inits wet condition a cross-sectional structure with cavities the fiber issegmented in the longitudinal direction by dividing walls the fibercomprises an absorbent polymer.
 2. The cellulose fiber according toclaim 1, wherein the fiber has a cross-section in the dry condition thatis multilobal and/or is irregularly grooved.
 3. The cellulose fiberaccording to claim 1 or 2, wherein the fiber has a width and a length,wherein the fiber comprises dividing walls and wherein the distancebetween the dividing walls is between 0.3 and 3 times the fiber widthaveraged over the fiber length.
 4. The cellulose fiber according toclaim 1 or 2, wherein the fiber comprises 5% by weight to 50% by weightof the absorbent polymer based on underivatized cellulose.
 5. Thecellulose fiber according to claim 1 or 2, wherein the absorbent polymeris carboxymethylcellulose.
 6. The cellulose fiber according to claim 1or 2, wherein said cellulose fiber is present in the form of a staplefiber.
 7. The cellulose fiber according to claim 1 or 2, wherein thefiber has a titer of 0.5 dtex to 8 dtex.
 8. The cellulose fiberaccording to claim 1 or 2, wherein the fiber has a length which is from2 mm to 80 mm.
 9. The cellulose fiber according to claim 1 or 2, whereincharacterized by a water retention capacity of at least 300%.
 10. Acylindrical tampon obtained from the cellulose fiber according to claim1 or 2, wherein said cylindrical tampon has a mass of 2.72 g, a lengthof 44 mm, a diameter of 13 mm and a Syngina value of at least 5.2 g/g.11.-17. (canceled)
 18. The regenerated cellulose fiber according toclaim 1 or 2 for use in absorbing products, sanitary products, inparticular tampons, sanitary aids for incontinence, sanitary napkins andpanty liners, packaging of foodstuff, in particular for meat products,papers, in particular filter papers, clothes and wound dressings. 19.The cellulose fiber according to claim 3, the distance between thedividing walls is between 0.5 and 2 times the fiber width averaged overthe fiber length.
 20. The cellulose fiber according to claim 4, whereinthe fiber comprises 15% by weight to 40% by weight of the absorbentpolymer based on underivatized cellulose.
 21. The cellulose fiberaccording to claim 4, wherein the fiber comprises 20% by weight to 30%by weight of the absorbent polymer based on underivatized cellulose. 22.The cellulose fiber according to claim 7, wherein the fiber has a titerof 1.3 dtex to 6 dtex.